1. Field of Invention
The present invention relates generally to actuators. More particularly, the present invention relates to a coil design which enables an actuator such as a linear motor to provide a net force in along both an x-axis and a z-axis.
2. Description of the Related Art
For many machines or instruments such as photolithography machines which are used in semiconductor processing, space is often at a premium. The lack of available space often forces components to be sized as compactly as possible. As a result, restricting the size of a component that is arranged to provide a particular force or motion allows the space in an overall machine to be efficiently utilized.
Many machines include linear motors which may be used to provide a force that is used to drive an object or a structure, e.g., a stage of a photolithography machine. FIG. 1 is a diagrammatic representation of a typical linear motor. A linear motor 100 includes a magnet structure 102 and a coil structure 104. Often, linear motor 100 is sized such that a dimension along an x-axis 106a and a dimension along a y-axis 106b are significantly larger than a dimension along a z-axis 106c. When a current is applied to coil 104, a force, i.e., a non-zero net force, is generated along x-axis 106a. 
FIG. 2a is a diagrammatic cross-sectional representation of a symmetric linear motor. A linear motor 200 includes magnets 202 and coils 206, 207. Coil 206 includes sections 204a, 204b, while coil 207 includes sections 204c, 204d. Sections 204a, 204b are arranged to produce force in an x-direction 206a, while sections 204c, 204d are arranged to produce force in a z-direction 206c. As shown in FIG. 2b, when current is applied to coils 206, 207 in a y-direction 206b, magnetic flux 208c is generated in x-direction 206a, and magnetic flux 208a, 208b is generated in z-direction 206c. Since force that is generated by linear motor 200 is generally perpendicular to both applied current and magnetic flux 208, coil 206 produces a force in x-direction 206a. Coil 207 is capable of generating force in z-direction 206c. However, since magnetic flux 208a is directed in a substantially opposite direction from magnetic flux 208b, the net force in z-direction 206c is effectively zero.
Since linear motor 200 effectively only produces a non-zero net force in x-direction 206a, linear motor 200 may generally only be used to apply force on an object, as for example a stage assembly (not shown), in x-direction 206a. In order for the object to be moved in z-direction 206c, an additional linear motor which is arranged to apply a force substantially only in z-direction 206c generally must also be coupled to object. While the use of linear motor 200 and an additional linear motor may be effective in allowing an object to move in both x-direction 206a and z-direction 206c, the use of the additional linear motor may not always be possible due to space constraints within an overall system. Further, the use of an additional linear motor may cause issues associated with the addition of mass to the overall system, and the generation of heat within the overall system. As will be appreciated by those skilled in the art, additional mass may cause vibrations within the overall system, while additional heat may adversely affect the performance of various components, e.g., sensors, within the overall system.
A planar motor, i.e., a motor with a substantially flat plate of magnets and coils, is arranged to provide force in an x-direction and a z-direction. Hence, a single planar motor may be used in lieu of two linear motors to provide a non-zero net force in an x-direction and a z-direction. However, a planar motor is generally more complicated to control than a linear motor. Further, since many systems are arranged to use linear motors, the use of a planar motor instead of one or more linear motors may be impractical.
Therefore, what is desired is a method and an apparatus which enables a non-zero net force to be efficiently applied in an x-direction and a z-direction. That is, what is needed is a method and an apparatus which allows a single linear motor to be used to apply non-zero net forces along an x-axis and a z-axis.